1. Field of the Invention
This invention lies in the field of liquid and gaseous fuel burning. More particularly, this invention concerns fuel burning apparatus in which the design of the burner and control of the fuel and air or oxidant supply is separately controllable for primary, secondary and tertiary air or oxidant, so as to maintain a minimum value of NOX in the effluent gases.
2. Description of the Prior Art
The burning of fuels, however it is accomplished in burners, as they are known in the art of fuel burning, is productive of oxides of nitrogen (NOX) in normal operation. Such oxides of nitrogen as are produced in combination with olefininc hydrocarbons, which may be present in the atmosphere, constitute a major source of smog.
Smog, while not necessarily lethal, is recognized universally as potentially damaging to animal tissue. Consequently, several limitations on the NOX content of stack gases vented to the atmosphere as a result of fuel burning, have been imposed by various governmental authorities and agencies.
The prior art is best represented by U.S. Pat. No. 4,004,875. This patent has been the basis of a wide appliacation of low NOX burners in the natural gas field. Scores of burners, which are based on this patent, are in commercial service, where they have a suppressed NOX as intended. However, the optimum operation of the prior patent has been for fixed rates of fuels burning, where a good balance can be provided between the primary and secondary air or oxidant supplies to a first combustion chamber and a supply of additional tertiary air or oxidant downstream of the first combustion chamber.
The weakness of the prior design is that for one condition of furnace draft or firing rate the operation is ideal. However, when the firing rate changes significantly, such as from 100% to 80%, as is typical of daily process heater firing, there is difficulty in maintaining NOX suppression. The reason for this is that, at reduced firing rate, the furnace draft remains constant or approximately so, and increased air-to-fuel ratios destroy the less-than-stoichiometric burning zone prior to tertiary air delivery, which results in less-than-optimum NOX reduction plus higher than desirable excess air.
What is required is a burner which provides means for correction for any condition of firing such as might be required when the furnace draft remains substantially constant, as changes in firing rates are made. If such corrections can be made, the result is continuation of NOX suppression and maintenance of optimum excess air for high thermal efficiency. In the prior art burner there is no control of the tertiary air which is caused to flow by furnace draft, while the primary and secondary air also flow for the same reason.
The total air flow will vary as the square root of the furnace draft. Thus, only one rate of fuel burning or firing rate, at a condition of furnace draft, will provide the required excess air and NOX suppression. This would seem to indicate that control of the air flow would provide some benefit. What is not immediately evident is that the air entry control must be proportionately controlled for maintenance of a less-than-stoichiometeric burning zone prior to the entry of tertiary air to the less-than-stoichiometric gases, for completion of fuel burning, plus preferred excess air when firing rate is caused to vary. If the conditions, as outlined, are maintained, there is a suitable NOX suppression in any condition of draft and firing rate, and the furnace excess air remains best for high thermal efficiency. This is to say that control of primary, secondary and tertiary air must be proportional and simultaneous for best and most assured operation in all firing conditions.